Neptunian (adj.)

1650s, "pertaining to the god Neptune;" 1794 in the geological sense, referring to certain features (later confirmed as volcanic) believed to be formed or deposited by actions of water, from Neptune + -ian. Usually opposed in the latter sense to volcanic or plutonic.  "A most violent discussion in regard to this subject was carried on, during the latter third of the eighteenth century, by geologists and theologians" [Century Dictionary]. As a noun meaning "inhabitant of the planet Neptune" it is recorded from 1870.